12.3 Radiographic Testing (RT): Acceptance Criteria, Film Interpretation & ASME V/API 650

Fundamentals of Radiographic Testing (RT) per ASME Section V, Article 2

Radiographic Testing (RT) is the historic volumetric inspection standard utilized during the shop fabrication, field erection, and post-repair alteration of aboveground atmospheric storage tanks. Governed by ASME Section V, Article 2 as referenced by API Standard 650 (Section 8.1) and API Standard 653 (Section 12.1), radiography provides a permanent, verifiable visual record of the internal volumetric integrity of full-penetration butt-welded joints.

+-------------------------------------------------------------------------+
|                    RADIOGRAPHIC EXAMINATION SETUP                       |
|                                                                         |
|                          Radiation Source                               |
|                      (X-Ray Tube or Ir-192)                             |
|                                 *                                       |
|                                /|\                                      |
|                               / | \  Source-to-Film                     |
|                              /  |  \ Distance (SFD)                     |
|                             /   |   \                                   |
|                            /    |    \                                  |
|               [IQI]       /     |     \                                 |
|               +---+      /      |      \                                |
|   ============|===|=====+=======+=======+============================   |
|   Base Metal  +---+    /   WELD BEAD     \                   Base Metal |
|   (Steel Plate)       /   (Discontinuity  \                  (Steel)    |
|   -------------------/     absorbs less    \-------------------------   |
|                     /       radiation)      \                           |
|                    +-------------------------+                          |
|                    |    Radiographic Film    |                          |
|                    | [Lead Screen / Cassette]|                          |
|                    +-------------------------+                          |
+-------------------------------------------------------------------------+

Mechanism of Differential Absorption

Radiography operates upon the principle of differential absorption of penetrating electromagnetic radiation. High-energy X-ray or gamma-ray photons pass through the steel weldment. The fraction of radiation transmitted through the material depends exponentially on metal density, atomic number, and through-wall thickness according to the Beer-Lambert law:

$I = I_0 \cdot e^{-\mu \cdot x}$

Where:

• $I_0$ is the incident radiation intensity.
• $I$ is the transmitted intensity reaching the film.
• $\mu$ is the linear attenuation coefficient of the material.
• $x$ is the total penetrated thickness.

Where volumetric discontinuities exist (such as slag inclusions, gas porosity, incomplete joint penetration, or cracks), the effective thickness $x$ of solid steel is reduced. Consequently, fewer photons are absorbed, and a higher intensity of radiation reaches the photographic emulsion. Upon chemical processing, these localized areas of increased exposure develop into darker (higher optical density) images against the lighter background of solid metal.

Radiation Sources: Industrial X-Ray vs. Gamma Radioisotopes

Two primary radiation sources are utilized in tank inspection:

+-------------------------------------------------------------------------+
|                   X-RAY VS. GAMMA RADIOISOTOPE COMPARISON               |
+-----------------------+-----------------------+-------------------------+
| Feature               | Industrial X-Ray      | Iridium-192 (Ir-192)    |
+-----------------------+-----------------------+-------------------------+
| Generation Method     | High-voltage vacuum   | Spontaneous radioactive |
|                       | tube (Electrical)     | decay (Nuclear isotope) |
| Energy Spectrum       | Continuous / Variable | Discrete gamma peaks    |
|                       | (e.g., 100 to 300 kV) | (~0.31, 0.47, 0.60 MeV) |
| Steel Thickness Range | Up to 1.5 in. (38 mm) | 0.25 to 2.5 in. (6-65 mm|
| Half-Life             | N/A (Switch off)      | 73.8 days               |
| Portability / Access  | Heavy, needs power    | Extremely compact       |
+-----------------------+-----------------------+-------------------------+

• Industrial X-Ray Tubes: Electrically generated radiation where electrons accelerated across a high potential strike a tungsten target. The inspector can adjust kilovoltage (kV) to control penetrating energy (contrast) and milliamperage (mA) to control beam intensity. X-rays provide small focal spots ($< 1.0\text{ mm}$ to $3.0\text{ mm}$), yielding exceptional geometric sharpness and high contrast on thinner shell plates.
• Gamma-Ray Radioisotopes:
  • Iridium-192 ($^{192}\text{Ir}$): The universal workhorse for field tank erection and repair. Emit gamma photons with energies ranging from $0.3$ to $0.6\text{ MeV}$, with a half-life of 73.8 days. It efficiently interrogates carbon steel plate thicknesses from $0.25\text{ in.}$ to $2.5\text{ in.}$ ($6\text{ mm}$ to $65\text{ mm}$). Its compact exposure device (camera) requires no external power source, enabling access around scaffolding, nozzles, and floating roofs.
  • Cobalt-60 ($^{60}\text{Co}$): Emits high-energy gamma rays ($1.17\text{ MeV}$ and $1.33\text{ MeV}$) with a half-life of 5.27 years. Due to its high penetrating power, it is reserved for very thick steel sections ($1.5\text{ in.}$ to $7.0\text{ in.}$ / $38\text{ mm}$ to $175\text{ mm}$), but suffers from lower radiographic contrast on standard tank shell thicknesses.

Image Quality Indicators (IQIs / Penetrameters)

To verify that a radiograph possesses acceptable sensitivity, image contrast, and spatial resolution, an Image Quality Indicator (IQI)—historically called a penetrameter—must appear on every production film.

IQI Types per ASTM and ASME Standards

ASME Section V Article 2 recognizes two primary IQI designs:

1. Wire-Type IQIs (ASTM E747): A plastic envelope containing a series of six parallel wires of graduated diameters. Four standard sets are manufactured:
   • Set A: Wire diameters $0.0032\text{ in.}$ to $0.010\text{ in.}$ ($0.08\text{ mm}$ to $0.25\text{ mm}$)
   • Set B: Wire diameters $0.010\text{ in.}$ to $0.032\text{ in.}$ ($0.25\text{ mm}$ to $0.81\text{ mm}$)
   • Set C: Wire diameters $0.032\text{ in.}$ to $0.100\text{ in.}$ ($0.81\text{ mm}$ to $2.54\text{ mm}$)
   • Set D: Wire diameters $0.100\text{ in.}$ to $0.320\text{ in.}$ ($2.54\text{ mm}$ to $8.13\text{ mm}$) The wire IQI is placed across the weld bead so that the wires lie perpendicular to the weld seam. The radiograph is verified by clearly identifying the continuous length of the specified essential wire across the weld and heat-affected zone.
2. Hole-Type IQIs (ASTM E1025 / ASME): A rectangular plaque of radiographically similar material having a nominal thickness ($T$) equal to $2%$ of the weldment thickness. The plaque contains three drilled holes with diameters of $1T$, $2T$, and $4T$ (where $T$ is plaque thickness). Standard radiography requires the $2-2T$ quality level (a $2%$ plaque thickness where the $2T$ diameter hole must be clearly resolved on film).

+-------------------------------------------------------------------------+
|                       IQI DESIGN ARCHITECTURE                           |
|                                                                         |
|      ASTM E747 WIRE-TYPE IQI               ASTM E1025 HOLE-TYPE IQI     |
|      +-----------------------+             +-----------------------+    |
|      | |   |   |   |   |   | |             |   +---+   +---+       |    |
|      | |   |   |   |   |   | |             |   |4T |   |1T |       |    |
|      | |   |   |   |   |   | |             |   +---+   +---+   (T) |    |
|      | |   |   |   |   |   | |             |       +---+           |    |
|      | W1  W2  W3  W4  W5  W6|             |       |2T | (Essential|    |
|      +-----------------------+             |       +---+  Hole)    |    |
|       (Essential Wire Must Be              +-----------------------+    |
|         Clearly Continuous)                 (Plaque Thickness = 2% t)   |
+-------------------------------------------------------------------------+

IQI Placement Rules and Film-Side Requirements

ASME Section V Article 2 Paragraph T-277 establishes strict placement rules:

• Source-Side Placement (Standard): The IQI must be positioned on the source side of the weld facing the radiation source.
• Film-Side Placement (Exception): If physical access prevents placing the IQI on the source side (e.g., an in-service closed tank or inaccessible piping spool), the IQI may be positioned on the film side.
• Mandatory Lead Letter "F": Whenever an IQI is placed on the film side, a lead letter "F" (at least $1/2\text{ in.}$ / $13\text{ mm}$ high) must be placed immediately adjacent to or directly on the IQI. A more stringent essential wire diameter or smaller hole size is required on the film side to compensate for the reduction in geometric enlargement.
• Shim Placement: When inspecting double-welded butt joints with weld reinforcement, the IQI cannot rest directly on the uneven weld crown without distorting sensitivity. Shims of radiographically similar material must be placed beneath the IQI so that the total steel thickness under the IQI matches the nominal plate thickness plus the total weld reinforcement.

Radiographic Film Density and Quality Standards per ASME Section V

Once a radiograph has been exposed and chemically processed, the degree of film darkening—termed optical density ($D$)—must be quantified using a calibrated transmission densitometer.

Optical density is defined mathematically as the common logarithm of the ratio of incident light intensity ($I_0$) to transmitted light intensity ($I$):

$D = \log_{10}\left(\frac{I_0}{I}\right)$

For example, an optical density of $D = 2.0$ means that only $1/10^2 = 1/100$ ($1%$) of incident light penetrates the film, while $D = 4.0$ transmits only $1/10^4 = 0.01%$ of incident light.

+-------------------------------------------------------------------------+
|             ASME SECTION V ARTICLE 2 FILM DENSITY THRESHOLDS            |
+-----------------------------------+-------------------------------------+
| Exposure Configuration            | Allowable Transmitted Density Range |
+-----------------------------------+-------------------------------------+
| Single Film: X-Ray Source         | 1.8 Minimum  to  4.0 Maximum        |
+-----------------------------------+-------------------------------------+
| Single Film: Gamma-Ray Source     | 2.0 Minimum  to  4.0 Maximum        |
| (Ir-192 / Co-60)                  |                                     |
+-----------------------------------+-------------------------------------+
| Composite Viewing of Multiple Film| 1.3 Minimum (each individual film)  |
| (Double-film viewing)             | 2.6 Maximum (composite viewing)     |
+-----------------------------------+-------------------------------------+

Density Limitations per ASME Section V Article 2 (T-282.1)

• Single Film - X-Ray: Minimum transmitted optical density through the weld and heat-affected zone is 1.8; maximum allowable density is 4.0.
• Single Film - Gamma-Ray: Minimum transmitted optical density is 2.0; maximum allowable density is 4.0.
• Composite Viewing: When two films are exposed simultaneously in a single cassette and viewed together under transmitted light, each individual film must have a minimum density of 1.3, and the composite density cannot exceed 2.6.

Allowable Density Variations Across the Area of Interest

Under ASME Section V Article 2 Paragraph T-282.2, the density measured anywhere through the weld or adjacent heat-affected zone cannot vary by more than $-15%$ or $+30%$ from the density measured through the essential wire of the wire IQI (or beside the hole-type plaque). Any film exceeding these bounds must be re-exposed.

Backscatter Radiation Check (The Lead Letter "B")

During exposure, radiation can scatter off concrete floors, structural steel, and backstop walls, striking the film from behind (backscatter). Backscatter fogs the film, severely degrading image contrast and masking fine cracks.

• Verification Procedure (Paragraph T-284): A lead letter "B" (minimum dimension $1/2\text{ in.}$ / $13\text{ mm}$ high and $1/16\text{ in.}$ / $1.6\text{ mm}$ thick) must be affixed to the back of each film holder or cassette.
• Acceptance Standard: If a light (low density) image of the letter "B" appears on a darker background of the processed radiograph, protection against backscatter is inadequate, and the radiograph must be rejected and re-shot with additional lead backing.

Film Interpretation of Weld Discontinuities

Radiographic film interpretation requires identifying and characterizing two-dimensional photographic projections of three-dimensional weld imperfections.

+-------------------------------------------------------------------------+
|               RADIOGRAPHIC DEFECT MORPHOLOGY COMPARISON                 |
+-----------------------+-----------------------+-------------------------+
| Defect Type           | Visual Film Appearance| Root Physical Cause     |
+-----------------------+-----------------------+-------------------------+
| Cracks                | Dark, fine, jagged,   | High shrinkage stress,  |
| (Planar)              | branching line with   | hydrogen embrittlement, |
|                       | sharp pointed tips    | solidification failure  |
+-----------------------+-----------------------+-------------------------+
| Incomplete Penetration| Straight, dark, sharp | Insufficient root gap,  |
| (Planar)              | continuous line along | low heat, excessive root|
|                       | weld centerline       | face dimension          |
+-----------------------+-----------------------+-------------------------+
| Lack of Fusion        | Dark, thin, very      | Cold lapping, improper  |
| (Planar)              | straight line along   | electrode manipulation, |
|                       | joint bevel edge      | oxides on bevel face    |
+-----------------------+-----------------------+-------------------------+
| Slag Inclusions       | Dark, elongated,      | Entrapped vitreous flux,|
| (Volumetric)          | irregular or jagged   | improper interpass      |
|                       | shapes (wagon tracks) | grinding and cleaning   |
+-----------------------+-----------------------+-------------------------+
| Porosity              | Dark, circular or oval| Gas entrapment (H2, N2, |
| (Volumetric)          | spots with smooth,    | CO2) during weld pool   |
|                       | rounded boundaries    | rapid solidification    |
+-----------------------+-----------------------+-------------------------+
| Tungsten Inclusions   | Characteristic bright | GTAW tungsten electrode |
| (High Density)        | white spots (absorbs  | touched to molten puddle|
|                       | more radiation)       | or eroded               |
+-----------------------+-----------------------+-------------------------+

Planar Imperfections

1. Cracks: Manifest as dark, irregular, narrow lines that meander or branch, terminating in sharp, high-stress tips. They may be longitudinal (along the weld center or HAZ) or transverse (across the weld bead). Cracks represent catastrophic brittle fracture initiators.
2. Incomplete Joint Penetration (IP / LOP): Occurs when the weld metal fails to extend through the full root thickness. It appears as a distinct, dark, continuous or intermittent straight line running precisely along the center of the root pass with sharp parallel edges.
3. Incomplete Sidewall Fusion (IF / LOF): Occurs when the weld metal fails to fuse completely with the base metal bevel or an adjacent weld bead. Appears as a very straight, dark, thin line running along the weld boundary, frequently accompanied by a straight edge and a wavy opposite edge.

Volumetric Imperfections

1. Slag Inclusions: Vitreous non-metallic solids entrapped in the weld puddle. They project as dark, irregularly contoured indications. When trapped along both sides of a root pass, they form parallel double lines known colloquially as "wagon tracks".
2. Gas Porosity: Gas bubbles trapped during solidification. They appear as well-defined, smooth, dark rounded spots. They may appear as uniformly scattered porosity, concentrated cluster porosity, or tubular "wormhole / piping" porosity.

Acceptance Criteria: API 650 Section 8.1 vs. ASME Section VIII UW-51 & UW-52

Radiographic acceptance standards depend on whether the examination is classified as Full Radiography or Spot Radiography.

1. Full Radiography (ASME Section VIII UW-51 & API 650 Section 8.1.1)

Full radiographic examination imposes the most stringent mechanical integrity standards:

• Planar Imperfections: Zero Tolerance. Any crack, zone of incomplete fusion, or incomplete joint penetration is strictly unacceptable regardless of length.
• Elongated Inclusions (Slag): Elongated inclusions are unacceptable if their length exceeds:
  • $1/4\text{ in.}$ ($6\text{ mm}$) for plate thickness $t \le 3/4\text{ in.}$ ($19\text{ mm}$).
  • $1/3 t$ for plate thickness $3/4\text{ in.} < t \le 2-1/4\text{ in.}$ ($19\text{ mm} < t \le 57\text{ mm}$).
  • $3/4\text{ in.}$ ($19\text{ mm}$) for plate thickness $t > 2-1/4\text{ in.}$ ($57\text{ mm}$).
  • In addition, any group of aligned slag inclusions having an aggregate length greater than $t$ in a weld length of $12t$ is unacceptable, unless adjacent inclusions are separated by at least $6L$ (where $L$ is the length of the longest inclusion).
• Rounded Indications (Porosity): Evaluated strictly against the standardized radiographic porosity charts in ASME Section VIII Div 1, Appendix 4. Welds exceeding the allowable concentration or maximum pore size shown in the charts must be cut out and repaired.

2. Spot Radiography (ASME Section VIII UW-52 & API 650 Section 8.1.2)

Spot radiography is a quality control sampling method designed to verify welder performance and procedural consistency:

• Planar Imperfections: Any crack, lack of fusion, or incomplete penetration is unacceptable.
• Slag Inclusions: Allowable individual slag inclusion length is expanded up to $2/3 t$ (compared to $1/3 t$ under full RT).
• Rounded Porosity: Completely exempt. Rounded indications and porosity are not a factor in spot radiography acceptance; welds cannot be rejected for porosity under UW-52.

Spot Radiography Rules for Tank Repairs under API 653 Section 12.1

API 653 Section 12.1 dictates the exact quantity, distribution, and geometric locations of radiographs required when replacement shell plates, insert plates, and door sheets are installed in reconstructed or altered tanks.

+-------------------------------------------------------------------------+
|             API 653 SHELL REPAIR SPOT RADIOGRAPHY LOCATIONS            |
|                                                                         |
|          Existing Shell Plate                                           |
|    -------------------------------+---------------------------------    |
|    Horizontal Weld Seam           | [1 Spot in First 10 ft]             |
|    -------------------------------v---------------------------------    |
|                                   |                                     |
|                                   |  Replacement Shell Plate            |
|    Existing                       |  or Insert Plate                    |
|    Vertical                       |                                     |
|    Weld                           |                                     |
|         \                         |                                     |
|          v [100% RT at T-Junction]| [1 Spot in Each Vertical Joint]     |
|    =======+=======================+=================================    |
|           |   <- Min 2 in. ->     |                                     |
|    -------+-----------------------+---------------------------------    |
|          Lower Shell Course                                             |
+-------------------------------------------------------------------------+

Code-Mandated Spot Radiograph Distribution (Section 12.1.2)

1. Vertical Butt Joints in Replacement Shell Plates:
   • For replacement plates of nominal thickness $t \le 3/8\text{ in.}$ ($10\text{ mm}$), one spot radiograph must be taken in each vertical joint.
   • For replacement plates with $t > 3/8\text{ in.}$, one spot radiograph is taken in each vertical joint, with additional spots or $100%$ examination governed by material toughness group and operating stress per API 650 Section 8.1.2.
2. Horizontal Butt Joints:
   • One spot radiograph must be taken in the first $10\text{ ft}$ ($3\text{ m}$) of horizontal weld completed for each welder or welding operator.
   • Thereafter, one additional spot radiograph is required for each $150\text{ ft}$ ($45\text{ m}$) of horizontal weld seam (or fraction thereof).
3. T-Junction Weld Intersections (Mandatory 100% Radiography):
   • API 653 Section 12.1.2.3 enforces an unyielding rule: All intersections where new vertical joints intersect new horizontal joints, or where new joints intersect existing shell joints (T-junctions), must be 100% radiographed.
   • Coverage Requirement: The radiograph must capture the intersection and extend to include at least 2 inches (50 mm) of weld along each arm of the intersection (both the vertical seam and the horizontal seam).
4. Butt-Welded Insert Plates and Door Sheets:
   • All full-penetration butt-welded joints connecting replacement insert plates or door sheets into the shell must be radiographed in accordance with Section 12.1. When insert plates are installed in the lowest shell course or across the annular ring, $100%$ RT or PAUT of all butt welds is typically mandated.